Inductors and variable inductors are useful and/or required circuit elements in a variety of important applications and products. For example, inductors and variable inductors are necessary elements of many RF-wireless products. In particular, they are used for matching and loading low noise amplifiers, power amplifiers and mixers, as well as for providing frequency- selective resonant circuits in variable-frequency oscillators in such RF-wireless products.
In a typical cellular telephone, inductors, and other "passive" components (e.g., capacitors and resistors) may occupy over 90 percent of the circuit-board space, and outnumber active devices by more than ten-to-one. As the functionality of such a telephone continues to be integrated into progressively fewer chips, the passive components that are not easily integrated have come to dominate the board-level design. Thus, it would be desirable to fabricate semiconductor-integrable passive components.
Several difficulties have impeded the development of such semiconductor-integrable passive components. Regarding inductors, important performance parameters for RF applications include the "quality factor," Q, (i. e., the relative absence of resistive losses) and obtaining a suitably high self-resonant frequency. Unfortunately, a performance improvement in one of such parameters is typically obtained at the expense of the other parameter. For example, increasing the size of an inductor will typically reduce resistive losses, but it will also lower its resonant frequency.
In the prior art, active circuits are typically used to "tune" or vary inductance in integrated circuits. Such an approach has several disadvantages, including degraded phase noise, relatively high power requirements and limited dynamic range.
A prior-art implementation of variable inductor that is described to be useful for silicon-based RF integrated circuit applications has been disclosed by Pehlke, et al in "Extremely High-Q Tunable Inductor for Si-Based RF Integrated Circuit Applications," 1997 IEEE Int'l Electron Devices Meeting, at 3.4.1-3.4.4., Washington, D.C., Dec. 7-10, 1997. That tunable inductor uses a variable-phase shifter to vary Q and inductance. While reported performance was good, the presence of the phase shifter is undesirable.